1. Field of the Invention
The present invention relates to a fuel cell system which generates electric power when fuel gas and oxidizing gas are supplied to fuel cells included in a fuel cell stack, and more particularly, to a technique for determining the presence or absence of a leaking cell having an abnormal electrolyte membrane.
2. Description of the Related Art
A method exists in the prior art for determining whether or not there is a leaking cell having an abnormal electrolyte membrane in fuel cells included in a fuel cell stack. According to this method, an output voltage of each fuel cell is measured in an activation overvoltage region while the pressure of fuel gas supplied to a fuel electrode is set to be higher than the pressure of oxidizing gas supplied to an oxidizing electrode in each fuel cell. If there is a fuel cell whose output voltage is equal to or less than a predetermined voltage, that fuel cell is determined to be a leaking cell. A leaking cell is a fuel cell in which gas flows through the electrolyte membrane from the fuel-electrode side to the oxidizing-electrode side.
In the prior art determination method, a pressure difference is generated between the fuel gas at the fuel electrode and the oxidizing gas at the oxidizing electrode of each fuel cell such that the pressure at the fuel electrode is higher than that at the oxidizing electrode. The generated pressure difference is maintained constant, and the resultant velocity of gas that flows from the fuel-electrode side to the oxidizing-electrode side is constant. Therefore, when, for example, the leakage is caused by a very small hole in the electrolyte membrane, it takes a long time for the output voltage to be reduced to a voltage equal to or less than the predetermined voltage, which occurs in response to a reduction in the oxygen partial pressure at the oxidizing electrode. Thus, it is difficult to determine the presence or absence of a leaking cell in a short time.